Process for removing silica from iron



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F mean-n61 k :sePA A'rom m n 40 O I O I) u o a lg m BENEFICIATED aeouceo3 IRON 9 (5.0 057. OR LESS) I Ill I! DECOMPOSITION 8P3 nzeeuznnon KF BF;(400 F.) (932 E) noLrEu KF -m-' E KF-BF3 5F; (B0F)3+6HF-3(BF3' 44 0) BPSH O +KFIKF'BF34- H2O HF REGENERATOR (248 800F.) STEAM OR SILICA GELPatented Nov. 21, 1950 PROCESS FOR REMOVING SILICA FROM IRON Vincent S.de Marchi, Chicago, Ill., assignor, by mesne assignments, to InlandSteel Company, Chicago,.lll., a corporation of Delaware Application May10, 1945, Serial No. 592,955

3 Claims. 1

This invention relates to a metallurgical process involving thetreatment of various materials with boron trifluoride (BFz) at anelevated temperature. I have found that boron trifluoride reacts attemperatures upwards of 200 F. with compounds of silicon, titanium,phosphorus, vanadium, columbium, arsenic, antimony, tantalum,molybdenum, tungsten, uranium, and the like, to form volat le compoundswhich can be swept away by additional amounts of boron trifluoride orother gases. Thus, said elements may be separated in volatile form fromother substances with which said elements may be admixed but which donot react with boron trifluoride at said reaction temperature to formvolatile compounds, for instance, iron, nickel, cobalt, copper, andoxides thereof. No boron is thereby introduced into the residual solidmaterial, for boron is continuously removed during the course of thetreatment in the form of gaseous boron oxyfluoride (BOFM.

The methods of the present invention are therefore applicable to thetreatment of ores and other material containing compounds of columbium,arsenic, antimony, tantalum, molybdenum, tungsten, uranium, and thelike, for the purpose of isolating these elements in the form of theirvolatile fluorides, which may then be decomposed and/or further treated,when it is desired to isolate these elements.

The methods of the present invention are further applicable to theseparation, from titaniumor vanadium-containing ores and material, ofthese two elements, which may then be isolated from their volatilefluorine compounds. Further, titanium and vanadium may also be removedfrom titaniun'ior vanadium-containing iron ores or other ferruginousmaterial when it is desired to prepare metallic iron free from titaniumor vanadium.

The methods of the present invention are of particular interest inconnection with the removal of silicic matter from any and allferruginous material, being especially applicable to the removal ofsilica from iron oxides and iron ores prior to the reduction of suchoxides or ores to metallic iron; to the removal of silica from metalliciron obtained from silica-containing ores or oxides by reduction at atemperature below the fusion point of the iron (sponge iron); and to theremoval of silicon that may be present in cast iron or the like in theform of iron sllicide. Complete separation of silicic matter may beeffected from finely or coarsely divided iron or other ierruginousmaterial, and, if desired, only superficial removal from larger particles.

, extracted with water.

It is therefore an important object of the present invention to providea method for separating from ores or other materials, including metals,silicon, titanium, phosphorus, vanadium, columbium, arsenic, antimony,tantalum, molybdenum, tungsten, uranium, and the like, by the action ofboron trifluoride and the generation of volatile fluorine compounds ofsaid elements that may be swept away from residual matter by additionalboron trifluoride or other gases,

A specific object of the present invention is to provide a method forseparation of silicic matter from ferruginous material by the action ofboron trifluoride.

Another specific object of the present invention is to provide acontinuous method of the nature indicated involving the step ofregenerating boron trifluoride.

Other and further objects and features of the present invention willbecome apparent from the following detailed description and'the appendedclaims.

For proceeding according to the present invention, boron trifluoride mayconveniently and inexpensively be generated from parts by weightanhydrous rasorite 242 parts by weight fluorspar (CaFz), and 181 partsby weight of silica (Si02) These materials react at about 1600 F.'toyield 120.parts by weight boron trifluoride, 37 parts by weight sodiumfluoride (NaF) and 360 parts by weight of a solid residue consisting ofCa0.SiOz. The course of the reaction is indicated by the followingequations:

Other B203 containing materials than rasorite may be substitutedtherefor.

The boron trifluoride, being'a gas, may be led directly to afiuorination chamber or to a gas holder. The solid products of thereaction are recovered separately and the sodium fluoride is Theresulting sodium fluoride solution is separated from the calciumsilicate by filtration and subjected to evaporation for recovering solidsodium fluoride which may be returned to the boron trifluoride generatorfor conversion into boron trifluoride.

Boron trifluoride generated as described hereinabove or derived fromother sources may be i applied to the separatory removal of .thehereinabove mentioned elements from mixtures of said el m nts orcompounds thereof with other eleand at 1400 F. to about 0.2%.

pauper mentsor compounds. In order-to illustrate the application ofboron trifluoride for the purposes disclosed, I have describedhereinbelow in detail my method for the removal of silica from flue dustcontaining 10.3% silica and reduced to metallic iron by the methoddisclosed and claimed in the application of John C. Kalbach, Vincent S.de Marchi and Frederick W. Sullivan, Jr., entitled Method of ReducingMetallic Oxides," Serial 549,914, filed August 1'7, 1944. As shown inthe flowsheet of Figure 1, the flue dust is initially subjected toreduction with hydrogen gas in fluidized condition at about 1100 F. Thereduced product is subjected to a conventional magnetic separation forreducing its silica content to 6%, the nonmagnetic fraction beingdiscarded. The magnetic fraction is subjected to the action of borontrifluoride, say, at 600 to 1000 F. for a time sufficient to reduce itssilica. content to 0.5% or less. The boron trifiuoride reacts with thesilica according to the equation:

Both boron oxyfiuoride (BOF); and silicon tetrafiuoride (Sim) are gases,and escape from the reaction chamber along with any unreacted borontrifluoride.

The reaction chamber may take the form of a rotary kiln, a fluidizingchamber such as described in the above identified copending Kalbach etal., application, an ordinary furnace in which the magnetic fraction ofthe reduced flue dust is spread in thin layers in open-topped shallowreceptacles, or any other suitable device for effecting intimate contactbetween the ferruginous material and the boron trifluoride.

Removal of silica by boron trifluoride can be eifected at temperaturesupward from 200 F. as illustrated by the following experiments carriedout with the above described magnetic flue dust fraction. This productwas placed in a steel boat disposed in the center of a Monel metalreaction tube heated to the desired temperature. Before passing borontrifiuoride through said tube, the tube was purged with nitrogen forabout minutes. Then boron trifluoride was allowed to flow through thetube for 100 minutes. Three series of experiments were carried out,respectively, at gas flow rates of 0.0057, 0.0085 and 0.0013 cubic feetper minute. In each series, the temperature was varied as between theexperiments. After the expiration of 100 minutes, nitrogen was allowedto flow for minutes through the tube to purge the system, and thematerial was allowed to cool. Samples were taken from the cooledmaterial, which was then magnetically concentrated by means of alaboratory electromagnet, and the sample as well as the magneticfraction obtained were analyzed to determine their silica contents. Theresults show that treatment with boron trifluoride for 100 minutes at200 F. reduced the silica content to about 5.5%; at 400 F. to about3.4%; at 600 F. to about 2%; at 800 F. to about 1.3%; at 1000* F. toabout 0.4%; at 1200 F. to about 0.3%;

Magnetic treatment of the product thus treated for 100 minutes mayfurther reduce the silica content by from 0.2 to 0.5%.

Treatment with boron trifluoride is preferably carried'out at 600 F. orhigher, since at lower temperatures the reaction between silica andboron trifluoride is rather slow. Complete removal of silica maybeeffected above 600? F., if the treatvided ferruginous material beingtreated.

Only a trace of boron will be found in the treated ferruginous material.Practically no iron is lost.

The extent of silica removal can be varied according to the puritydesired for the reduced product. For use in powder metallurgy, finelydivided iron should contain less than 1% silica, preferably less than0.4%. For use in charging the open hearth, reduced iron should containat most about 2% silica. A silica content of from 10 to 12% ispermissible in iron to be fed to the blast furnace.

As indicated in the fiowsheet of Figure 1, the ferruginous materialtreated with boron trifiuoride may be subjected to a magneticseparation, to remove small amounts of silica still present, which arethen discarded. Thebeneflciated reduced iron containing 0.5% or lesssilica may be briquetted and charged to the open hearth for theproduction of steel.

The waste gases, which contain 2 parts by weight silicon tetrafluorideand 1 part by weight boron oxyfluoride (BOFM, together with anyeventually unreacted boron trifluoride, may be regenerated by beingpassed through a preheater for heating to 1400 F. and then introducedinto a regeneration chamber for passage through boron oxide (B203) at atemperature of 1470 F. or higher. Regeneration of boron trifluoride fromsilicon tetrafiuoride takes place according to the following equation:

The silica obtained combines with the excess of boric oxide to formborosilicate glass (Pyrex) having an approximate composition of SiOz,12% B203, balance N820 and A1203.

The gaseous product issuing from the regeneration chamber includes borontrifluoride and boron oxyfluoride. This gaseous mixture is passedthrough a cooler for lowering the temperature to 300 F. At thistemperature boron oxyfluoride is decomposed into boric oxide and borontrifiuoride according to the equation:

The boron trifluoride obtained by decomposition of boron oxyfiuoride,together with the boron trifluoride obtained in the regeneration chamberby reaction between silicon tetrafluoride and boric oxide, is passed toa gas holder or directly to a fluorination chamber for treatment offurther amounts of ferruginous material. The boric oxide obtained bydecomposition of boron oxyfluoride in the cooler is transferred to theregeneration chamber for use in the regeneration of further amounts ofspent gas.

Another method for regenerating boron trifiuoride is shown in theflowsheet of Figure 2 illustrating the treatment of magneticallyconcentrated reduced iron containing 6% SiOz with boron trifluoride at.say. 600 to 1000 F. followed by an optional magnetic separation yieldinga beneficiated reduced iron containing less. than 0.5% silica. The wastegases, which also contain 2 parts by weight silicon tetrafluoride and 1part by weightboron oxyfluoride together with any eventually unreactedboron tetrafluoride, are passed to a reaction chamber for passagethrough a mixture of molten sodium fluoride and molten boric oxide at1650 F. The reaction occurring in the regeneration chamber isillustrated by the following equation:

Preferably some calcium fluoride is added to the regeneration chamber toreplace fluoride losses.

The gases issuing from the regeneration chamber are passed through acooler for cooling to 300 F. with resultant decomposition of boronoxyfluoride to form boric oxide and boron trifluoride, as in the methodillustrated in the flowsheet oi Figure 1. The boron trifluoride isreused for beneflciating further amounts of ferruginous material, andthe boric oxide is returned to the regeneration chamber.

The solid products obtained from the regeneration chamber are treatedwith water, acidified water, or a dilute acid, and filtered, yielding,as a solid residue, a mixture of silica and calcium silicate togetherwith a solution of sodium fluoride and boric acid. This solution isevaporated, if desired, to dryness, the dry solids obtained arecalcined, and the evaporated solution or the solids obtained therefromare returned to the regeneration chamber for reuse.

Still another regeneration process is shown in the flowsheet of Figure 3illustrating the treatment of magnetically concentrated reduced ironcontaining 6% silica with boron trifluoride at, for instance, 600 F.followed by magnetic separation yielding a beneficiated reduced ironcontaining 0.5% or less silica. The waste gases containing 2 parts byweight silicon tetrafluoride, 1 part by weight boron oxyfluoride andsome unreacted boron trifluoride are passed through a cooler for coolingto about 450 F. The cooled gases are passed to a regenerator forreaction at 400 F. with molten potassium hydrofluoride (KRHF) accordingto the following equations:

The KFBFa is removed to a decomposition I chamber for decomposition at,say, about 932 F.,

into potassium fluoride and boron trlfluoride according to the followingequation:

The regenerated boron trifluoride is utilized for removal of silica fromfurther amounts of ferruginous material, while the potassium fluoride isreturned to the regeneration chamber.

The silicon tetrafluoride after passing through the regenerating chamberis admixed with water and the resulting mixture is passed to a hydrogenfluoride regeneration chamber where, at a temperature of, for instance,248 to 800 F., the silicon tetrafiuoride is decomposed with steamaccording to the following equation:

Other methods for the regeneration of boron trlfluoride may also beutilized.

The preceding detailed description has referred particularly to thebeneficiation of flue dust. It should be understood that thisdescription has been given merely by way of example. The removal ofsilica by means of boron trifluoride may be applied to flue dust priorto the reduction thereof, and may be applied to other iron ores andoxides as well, both prior to and subsequent to reduction. The methodsof this invention ma also be applied to the removal of elemental siliconor silicon present in ferruginous material in the form of silicides.Further, although rapid complete removal of silicic matter fromferruginous material requires that the silicic matter be easilyaccessible to the boron trifluoride, as in the case of finely divided orporous ferruginous material, yet superficial removal of silicic matterfrom larger granules or particles of ferruginous material may besuccessfully accomplished in relatively short time, or a completeremoval by prolonged exposure to boron In other words, the methods ofthe present invention are applicable to ferruginous materials of allkinds and containing silicon in various forms in chemical combination ormechanical admixture or conglomeration. Boron trlfluoride may be used inpure or relatively pure form or in admixture with other gases suchas'nitrogen, carbon monoxide or air. I

A very simple and inexpensive way of carrying out the method of thisinvention comprises incorporating with the material to be treated afiuoborate of the general formula MFBF: (M=Metal), such as CaF2.2BFs,NaF.BFa, NH4BF3 or the like. The resulting mixture is heated to atemperature high enough to decom pose the fluoborate into BF3 and ametal fluoride (MF), say, 900 F. or higher. The boron trifluoride thenliberated will react with the silicic matter or other substance to beseparated, and the volatile fluoride then formed can be swept away. Whenthis method is used, the solid residue obtained will of course containthe metal fluoride component of the fluoborate originally admixed withthe material being treated, but the presence of such a metal fluoride isnot always objectionable.

As pointed out hereinabove, boron trifluorideis applicable, not only tothe separation of silicic 3P205+ 15BF3 =6PF5 5 (BOF) 3 The above datawere obtained by treatment of magnetically concentrated reduced fluedust by me'hods similar to those disclosed hereinabove for the removalof silicic matter from said flue dust. Phosphorus remo al is effected attemperatures as low as 700 F. and also at temperatures above 1100 F.,although the efliciency is notas great as at from l000 to 1100 F.

The phosphorus fluoride obtained may be detreated with boron trifluorideto form a volatile 1 fluoride (boiling point 543 F), in accordance withthe following equation:

Hence, treatment with boron trifluoride above 550 F. may be used toremove titanium from the titaniferous ores and concentrates thereoffound in New York, Wyoming, Canada, Norway and Sweden, for instance, theiron ore from Cranberry, North Carolina, containing 0.17% T: or itsconcentrate containing 0.20% TiOa; the magnetite from Iron Mountain,Wyoming, containing 21.9% T102; the iron ore from Iron Mine Hill,Cumberland, Rhode Island, containing 9.30% TiO: or its concentratecontaining 16.8% T102; the iron ore from Horse Sign Butte, Curry County,Oregon. containing 4.85% TiOz or its concentrate containing 1.4% T102;or the magnetite beach sand from Sunnybrook, Connecticut,

containing 0.83% TiO: or its concentrate con- 'taining 1.53% T102.

Treatment with boron trifluoride is likewise capable of removingvanadium from ferruginous Fluorination is carried out at a temperaturehigher than 400 F. The boiling point of vanadium pentafluoride is 232 F.The reaction products will be swe t away from the ferru inous materialby fresh fluorination gas and collected outside of the reaction chamber.

when it is desired to recover vanadium from its ores. a trea ment withboron trifiuoride may be applied to various vanadi m ores such a roastedpatronite: or camotite. KO2UO3.V2O5.3H2O: or vanadinite, 3Pba(VOOaPbC12;or roscoelite, HgK-dMgF'e) (AW) 8103):; or the various vanadium oresmentioned in the article by George 0. Argall in the Colorado School ofMines Quarterly. vol. 38, No. 4, October. 1943, p. 56.

Treatment with boron trifluoride is also operative to volatilize asfluorides tantalum and columbium present in ores such as roastedcolumbite or tantalite: molybdenum from ore such as molybdenite (Mose),wulfenite (PbMooi), and molybdite (M001) tungsten in ores such aswolframite (FeWOd scheelite (CaWOd ferberite (FeO.WO3)

other compounds thereof, by treatment of compositions containing saidelements, their oxides.

silicates or complex salts with boron trifluoride at a temperature abovethe boiling point of the fluorides of said elements. Such treatment maybe used either to remove said elements and compounds thereof frommaterial such as iron which it is desired to purify or else to isolateand recover said elements or compounds thereof.

Many details of composition and procedure may be varied within a widerange without departing from the principles of this invention, and itis, therefore, not my purpose to limit the pat ent granted on thisinvention otherwise than necessitated by the scope of the appendedclaims.

I claim as my invention:

1. The method of removing silicic matter from finely divided iron whichcomprises contacting said iron at a temperature of at least 600 F. withboron trifluoride for a time sufllcient to volatilize said silicicmatter in the form of silicon tetrafluoride, separating the resultinggaseous mixture from said iron, contacting the separated silicontetrafluoride at least 1450 F. with molten boric oxide to regenerateboron trifluoride, and contacting the regenerated boron trifluoride withadditional amounts of finely divided iron.

2. The process for separating silica from finely divided iron whichcomprises contacting said iron at a temperature of at least 600 F. withboron trifluoride to volatilize said silica in the form of silicontetrafluoride, separating the resulting silicon tetrafiuoride fromsaid-iron, contacting said silicon tetrafluoride with molten boric oxideat a temperature of at least 1470 F. to regenerate boron trifiuoride,and contacting additional amounts of said finely divided iron with saidregenerated boron trifluoride.

3. The method of reducing the silica content of finely divided ironwhich comprises contacting said iron with boron trifluoride at atemperature of at least 600 F., separating the resulting mixture ofsilicon tetrafiuoride and boron oxyfluoride from said iron, contactingsaid mixture of silicon tetrafluoride and boron oxyfluoride with moltenboric oxide at a temperature of at least l470 F. to convert said silicontetrafluoride into boron trifluoride, cooling said gaseous mixture todecompose said boron oxyfluoride and to form additional amounts of borontrifluoride,

and contacting additional amounts of said finelydivided iron with saidregenerated boron triiiuoride.

r V VINCENT S. m: MARCHI.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Num er Name 7 Date 1,532,072 Parsons et al Mar.21. 1925 2,163,232 Baldeschwieler June 20, 1939 2,293,939 FahrenwaldAug. 25, 1942 2,386,337 Moyer Oct. 9, 1945 OTHER REFERENCES

3. THE METHOD OF REDUCING THE SILICA CONTENT OF FINELY DIVIDED IRONWHICH COMPRISES CONTACTING SAID IRON WITH BORON TRIFLUORIDE AT ATEMPERATURE OF AT LEAST 600*F., SEPARATING THE RESULTING MIXTURE OFSILICON TETRAFLUORIDE AND BORON OXYFLUORIDE FROM SAID IRON, CONTACTINGSAID MIXTURE OF SILICON TETRAFLUORIDE AND BORON OXYFLUORIDE WITH MOLTENBORIC OXIDE AT A TEMPERATURE OF AT LEAST 1470*F. TO CONVERT SAID SILICONTETRAFLUORIDE INTO BORON TRIFLUORIDE, COOLING SAID GASEOUS MIXTURE TODECOMPOSE SAID BORON OXYFLUORIDE AND TO FORM ADDITIONAL AMOUNTS OF BORONTRIFLUORIDE, AND CONTACTING ADDITIONAL AMOUNTS OF SAID FINELY DIVIDEDIRON WITH SAID REGENERATED BORON TRIFLUORIDE.